Agriculture is a significant source of carbon dioxide (CO₂) and methane (CH₄) and is the dominant source of anthropogenic nitrous oxide (N₂O) emissions. Changes in agricultural land management practices that reduce overall greenhouse gas (GHG) emissions have been suggested to help mitigate climate change, but a better understanding of the timing, magnitude, and drivers of GHG fluxes is needed. Alfalfa agroecosystems may be significant sources of N₂O given their ability to increase N inputs through symbiotic N₂ fixation and frequent irrigation events that create conditions for hot moments of N₂O production. However, few studies have explored long-term N₂O emissions and their associated drivers in alfalfa ecosystems. We collected over 108,000 CO₂, CH₄ and N₂O soil flux measurements over four years using cavity ring-down spectroscopy from a conventional flood-irrigated alfalfa field in California, USA. This ecosystem was a consistent source of N₂O (annual mean: 624.4 ± 27.8 mg N₂O m^-2 yr^-1, range: 263.6. ± 5.6 to 901.9 ± 74.5 mg N₂O m^-2 yr^-1) and a small net sink of CH₄ (annual mean: -53.5 ± 2.5 mg CH₄ m^-2 yr^-1, range: -78.2 ± 8.8 to -31.6 ± 2.5 mg CH₄ m^-2 yr^-1). Soil CO₂ fluxes averaged 4925.9 ± 13.5 g CO₂ m^-2 yr^-1 and were greater than other alfalfa ecosystem estimates, likely driven by elevated temperatures and plant productivity throughout the growing season. Hot moments of N₂O emissions represented only 0.2% to 1.1% of annual measurements but were 31.6% to 56.8% of the annual flux. We found that both the magnitude and the contribution of N₂O hot moments to annual emissions decreased over time. Normalized difference vegetation index (NDVI), soil temperature, moisture, and O₂ were all significantly correlated with soil CO₂, N₂O, and CH₄ fluxes, although associations varied across both soil depth and timescales. Our results suggest that flood-irrigated alfalfa is a significant source of agricultural N₂O emissions, and that plant productivity and soil moisture effects on O₂ availability may modulate the net GHG budget of alfalfa agroecosystems.